Position-determining apparatus

ABSTRACT

An apparatus is provided for determining the position of a shaft that is rotatably displaceable about a longitudinal axis. The apparatus includes a magnet, first and second magnet-detecting devices that are arranged adjacent the shaft, and a third magnet-detecting device mounted on the shaft. A rotatably displaceable disk having the magnet mounted thereon is arranged to allow the magnetic field produced by the magnet to pass over and activate each of the magnet detection devices sequentially. A position-determining circuit connected to the magnet-detecting devices, measures the time interval that the magnet-detecting devices are sequentially activated by the magnetic field and applies the time so measured to a position-determining scheme. The position-determining scheme uses the time measured and the physical geometry of the apparatus to determine the position of the shaft.

BACKGROUND OF INVENTION

The present invention relates generally to position-determining devicesand more particularly to an apparatus for determining the absoluteposition of a rotating shaft.

It is a common problem to want to know the position of a device whoseposition is being controlled by an actuator or some other means. Forexample, in the controls industry, devices such as valves, each having avalve stem or valve shaft which is rotatable by an actuator, aretypically used to control the flow of liquids or various gassesassociated with the industrial process. In these applications, it is acommon need to know the precise absolute position of the controlledshaft or stem. This information allows for an improved understanding ofthe process and, subsequently, a more-accurate control of the process bya process control system.

A number of prior solutions are known for measuring or determining thisaforementioned position. One method is the use of linear variabledifferential transformers which can provide accurate positionalinformation. However, they require a mechanical linkage to translate thepositional information to a sensor and, additionally, consume arelatively high amount of power in their operation.

Slidewire, potentiometers, or other rotary transducers again require amechanical link to the controlled device and also have the disadvantageof a sliding electrical contact which can cause long-term unreliabilityas well as having a potential for producing arcing and/or sparking,precluding the use of these devices in volatile environments.

Hall effect transducers, as they are currently used, generally require amechanical linkage.

Additionally, all of the aforementioned devices and methods require theuse of analog-to-digital circuits that convert analog positionalinformation to the digital signals normally required by moderncomputer-controlled industrial process control systems.

Therefore, it is an object of the present invention to provide areliable position-determining apparatus that does not require amechanical linkage between the apparatus and a rotatable shaft.

It is a further object of the present invention to provide aposition-determining apparatus that has long-term reliability and iscost effective.

It is a further object of the invention to provide aposition-determining apparatus that exhibits the benefits of a greatlyreduced parts count when interfacing the resultant positional signals toa computer-controlled industrial process control system.

SUMMARY OF THE INVENTION

The apparatus of the present invention contemplates the use of aplurality of magnet-detecting devices, and a rotor disk having a magnetor a magnetic field source mounted thereon. The rotor disk is arrangedto allow the magnetic field produced by the magnet to pass over each ofthe magnet-detecting devices sequentially. The center axis of the rotordisk is aligned along the longitudinal axis of the shaft or valve stemwhose position is to be measured.

A first magnet-detecting device is mounted to the apparatus in a firstposition adjacent the rotatable shaft and along a rotational center lineof the rotor disk where the aforementioned magnet is located. The firstmagnet-detecting device is arranged to produce a first detection signalresponsive to the detection of the magnetic field. A secondmagnet-detecting device is mounted to the apparatus in a second positionalso adjacent the rotatable shaft but on an opposite side from the firstmagnet-detecting device. The second magnet-detecting device is alsopositioned along the rotational center line of the disk. The secondmagnet-detecting device is arranged to produce a third detection signalresponsive to the detection of the magnetic field. A thirdmagnet-detecting device is mounted to the rotatable shaft. The thirdmagnet-detecting device is also positioned along the aforementionedrotational center line of the disk between the first and the secondmagnet-detecting devices. The third magnet-detecting device is arrangedto be rotatably displaced along the rotational center line of the diskin direct relationship to the rotation of the shaft. The thirdmagnet-detecting device produces a second detection signal responsive tothe detection of the magnetic field.

The rotor disk allows the magnetic field from the magnet to pass overeach of the three magnet-detecting devices in sequence, as the rotordisk travels in a clockwise direction. As the disk is rotated, themagnetic field passes over and is coupled to the first magnet-detectingdevice, which produces the first detection signal. As the disk isfurther rotated, the magnetic field is uncoupled from the firstmagnet-detecting device. The magnetic field than passes over and iscoupled to the third magnet-detecting device, the magnet-detectingdevice associated with the valve shaft, producing the second detectionsignal. As the disk is further rotated, the magnetic field is uncoupledfrom the third magnet-detecting device. Upon further clockwise rotationof the disk, the magnetic field is coupled to second magnet-detectingdevice, producing the third detection signal. Finally, the furtherrotation of the disk will uncouple the magnetic field from the secondmagnet-detecting device.

A counting scheme is arranged to receive the first, the second, and thethird detection signals. The counting scheme starts a first countingsequence when the first detection signal is received. When the seconddetection signal is received, the first count is halted and a firstcount signal is generated. The counter is then initiated and a secondcounting sequence started. When the counting scheme receives the thirddetection signal, the second counting sequence is halted and a secondcount signal is generated.

The first and second count signals are read by a shaftposition-determining scheme whereby the first and the second countsignals are used to calculate the position of the shaft based on a ratiobetween the time measured by the count signals and the physical geometryin degrees between the first and second magnet-detecting devices. Theratio provides the location of the third magnet-detecting device and,therefore, the absolute position of the shaft along its longitudinalaxis.

Thus, there is provided an apparatus for determining the position of arotatable shaft and which offers long-term reliability and is costeffective in its operation. The present invention further benefits fromthe absence of the need to have sensed analog positional informationtranslated into digital information, which is typically required bymodern computer-controlled industrial process control systems.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention may be had by reading thefollowing detailed description in conjunction with the drawings inwhich:

FIG. 1 shows the position-determining apparatus of the present inventionas it would be used to advantage on a valve having a rotatable valveshaft or stem;

FIG. 2 shows the position-determining apparatus of the present inventionin an isometric view;

FIG. 3 shows a diagram of the geometric relationships among themagnet-detecting devices of the present invention; and

FIG. 4 shows the position-determining circuit of the present inventionused to calculate the position of the rotatable valve shaft or stem.

DETAILED DESCRIPTION OF THE INVENTION

A typical valve 10, where the present invention is used to advantage, isshown in FIG. 1. Valve 10 is comprised of a valve body 15, suitablyconnected on one end to a conduit 17 and on an opposite end to a conduit18. A valve shaft 11 extends through valve body 15 and includes anopening or orifice 14 extending through the shaft. The valve shaft 11 isdisposed to be rotatably displaceable along a longitudinal axis shown as12, in the directions shown as 13. Typically, the valve shaft 11controls the flow of a controlled medium such as a liquid, or a gas,from one side of the valve body 15 and conduit 17 to the opposite sideof the valve body 15 and conduit 18.

The method in which valve shaft 11 controls the flow of a controlledmedium will now be explained. This explanation will be made with theflow of the controlled medium moving in the direction shown by arrow 19,or from conduit 17 to conduit 18. As will be understood by those skilledin the art, the controlled medium can also flow in the oppositedirection, from conduit 18 to conduit 17. Valve shaft 11 will operate inthe same manner with flow in either direction and is not limitedthereto. The flow is controlled by rotating valve shaft 11 in eitherdirection 13. When valve 10 is in a closed position, a solid surface ofvalve shaft 11 is presented to the normal direction of medium flow.Rotational displacement of the valve shaft 11 will progressively movethe solid surface away from the medium allowing opening 14 toproportionally open from a partially-open position to a fully-openposition.

It will be understood by those skilled in the art that when the solidsurface of valve shaft 11 is presented to the controlled medium, theflow of the medium is effectively stopped from flowing through valveshaft 11 to conduit 18. Rotational displacement of the valve shaft 11thereby presenting opening 14 to the medium, allows the medium to flowthrough the valve shaft 11 and to conduit 18. The amount of flow acrossthe valve shaft 11 is controlled by the amount of opening 14 that ispresented to the medium. A small presentation allows only a small amountof flow to occur, while the full presentation of opening 14 would allowa maximum amount of the controlled medium to flow. Therefore, the amountof flow between conduit 17 and conduit 18 is directly proportional tothe rotational displacement of valve shaft 11. Valve shaft 11 can beeither manually rotated by hand, or connected to an actuating device(not shown) which can rotatably displace valve shaft 11 responsive topositioning signals from a process control system.

It is desirable within the environment of a process control system toknow at any given time the precise position of the valve shaft 11.Additionally, it is also desirable to be able to monitor the rotationaldisplacement of valve shaft 11 as an actuator moves or displaces thevalve shaft 11, for example, as when the flow of the controlled mediumis required to be increased or decreased under control of the processcontrol system. Further, many valve actuating devices require theprecise position of the valve shaft be known as it is displaced. Thisposition is normally communicated to the valve actuating device via afeedback signal. This feedback signal is used to gauge the progress ofthe valve shaft rotation in order to either increase or decreaserotation. This minimizes the over or under displacement of the shaft, oras it is more commonly known in the industry, the overshoot orundershoot, respectively.

The valve 10 just explained and shown by FIG. 1 is an example of theenvironment where the present invention can be used to advantage It willbe appreciated by those skilled in the art that the present inventioncan also be effectively used in other industrial control functions, suchas to control furnace dampers and is not limited thereto.

With continued reference to FIG. 1, and also with reference to FIG. 2,the apparatus for determining the position of a rotating shaft of thepresent invention is shown generally as 30. The apparatus 30 includesstationary magnet-detecting devices 31 and 32 mounted on respectivestems 34 and 35. Stems 34 and 35 are fixedly mounted on a stationaryplatform 38. A rotatable magnet-detecting device 33 is mounted to stem36, which is in turn mounted to valve shaft 11. As can be seen andunderstood, any rotatable displacement of valve shaft 11 will alsodisplace magnet-detecting device 33. Additionally, magnet-detectingdevices 31, 32, and 33 lie within the center line of a radial axis.Magnet-detecting devices 31, 32, and 33 are each comprised of a coil orany such device that is arranged to have a pulse induced in each coil asa magnet or a magnetic field source passes over each coil. Each of themagnet-detecting devices 31, 32, and 33 further includes a signal lead37, which is connected to the position-detection circuit of the presentinvention.

A rotor disk 40 is mounted in front of valve shaft 11 andmagnet-detecting devices 31, 32, and 33. Rotor disk 40 is constructed ofa ferromagnetic material and is mounted to a small electric motor 41 viathe electric motor's shaft 42. The longitudinal axis of electric motorshaft 42 and the concentric center of rotor disk 40 are aligned alongthe longitudinal axis 12 of valve shaft 11. Rotor disk 40 furtherincludes a magnet 43, mounted on a interior surface of rotor disk 40.Magnet 43 is located on rotor disk 40 adjacent its outer perimeter edgein a position where, as the rotor disk 40 is rotated, magnet 43 willpass over each of the magnet-detecting devices 31, 32, and 33. Themagnet 43 can be either a permanent magnet or an electromagnet of thetypes that are commonly known to those skilled in the art. Magnet 43,however, should be mounted to rotor disk 43 in a manner to allow themajority of its resultant magnetic field to be directed toward themagnetic detecting devices 31, 32, and 33.

Turning now to FIG. 3, a detailed explanation of the geometricrelationships involved in the operation of the present invention will bediscussed. As can be seen, FIG. 3 diagramatically shows the relationshipof the magnet-detecting devices to each other, as would be seen bymagnet 43 of rotor disk 40. Magnet-detecting devices 31, 32, and 33 alllie within the aforementioned radial axis center line shown as 60. Aswas previously explained, magnet-detecting devices 31 and 32 are fixedlymounted to the apparatus 30. Magnet-detecting device 33 is mounted tovalve shaft 11 and is arranged to traverse along the radial axis centerline 60. A rotation of valve shaft 11 along its longitudinal axis 12would be translated by travel of magnet-detecting device 33 along axis60 between magnet-detecting devices 31 and 32. Therefore, the locationor position of valve shaft 11, at any one time, can be calculated bymeasuring the rotational angle between magnet-detecting device 31 to 33providing a first angle 61 (α) and the rotational angle between lightdetecting devices 33 and 32 providing the second angle 62 (β). Sincemagnet-detecting devices 31 and 32 are fixed, the total range ofrotation can be expressed as:

    R=α+β

where "R" is the total range of rotation, "α" is the rotation in degreesbetween magnet detection devices 31 and 33, and "β" is the rotation indegrees between magnet detection devices 33 and 32. Thus, "R" representsthe physical geometry of the apparatus. As will be understood by thoseskilled in the art, the physical geometry of the apparatus of thepresent invention is arbitrary, with a constraint that "R" is less than360 degrees.

The apparatus of the present invention contemplates the use of time tomeasure the degree rotations of 61 and 62. This can be accomplished bymeasuring the time that magnet 43 travels in one direction sequentiallybetween magnet-detecting devices 31, 33, and 32. A ratiometricassociation of time to degrees of rotation can thus be expressed as:

    T.sub.1 /(T.sub.1 +T.sub.2)=α/α+β

where, T₁ is the time that magnet 43 travels between magnet-detectingdevices 31 and 33 and T₂ is the time that magnet 43 travels betweenmagnet-detecting devices 33 and 32.

Therefore, the absolute position of the shaft can be determined by aposition-determining calculation solving where along the radial centerline 60 and between magnet-detecting devices 31 and 32 magnet-detectingdevice 33 is located. The position of magnet-detecting device 33 can beexpressed as a percentage of shaft travel or "P" since it is physicallymounted to shaft 11. Therefore, the following position determiningalgorithm or formula can be used to find the position ofmagnet-detecting device 33 within the total range of rotation of theapparatus:

P=T₁ /(T₁ +T₂)

α=PR

β=R-α=(1-P)R

It can be appreciated from the above-defined position-determiningalgorithm that the absolute position of shaft 11 can thus be ascertainedby taking a time measurement between magnet-detecting device 31 and 33(T₁) and magnet-detecting device 33 and 32 (T₂) and including the timemeasurements T₁ and T₂ thus obtained, into the constants of the knownphysical geometry of the apparatus.

Turning now to FIG. 4, a position-determining circuit used to calculatethe position of the rotatable valve shaft 11 is shown. Theposition-determining circuit of the present invention includes trailingedge detection devices 81, 82, and 83 each having their respectiveinputs connected to the output of a respective magnet-detecting device31, 32, and 33. Devices 81, 82, and 83 are shown in this embodiment asschmitt trigger devices but any known device that produces a signal onthe transition of a negative-to-positive or positive-to-negative-goingsignal can be used. Each of the trailing edge detection devices 81, 82,and 83 has its respective output connected to NOR gate 85. As can beseen, an output signal from any of the devices 81, 82, and 83 will causean output signal from NOR gate 85. The output of NOR gate 85 isconnected to the Interrupt Request (IRQ) input of a microprocessor 70.The output of each of the devices 81, 82, and 83 is further connected torespective I/O ports PO1, PO2, and PO3 of device 70. A Read Only Memory(ROM) 71 is associated with microprocessor 70 and is used to store theratiometric calculating algorithm of the present invention and theoperating program or processing instructions used by the microprocessor70. A Random Access Memory (RAM) 72 is also associated withmicroprocessor 70 and is used as a memory store for the T₁ and T₂ countsand the digital representation of the position of the valve shaft 11.

It is contemplated that the present invention will be used with aprocess control system (not shown) that includes a Control System BUS 91that is connected to microprocessor 70 via communications BUS 92. Theprocess control system will from time to time poll microprocessor 70,requesting the transmission to the process control system of the storeddigital data representing the position of the valve shaft 11. However,it will be understood by those skilled in the art that microprocessor 70could also be connected to the local controller of a valve actuatingsystem, thereby providing feedback signal representing the valve'sposition as the valve shaft is rotated to a desired position. Further,ROM 71 and RAM 72 could also be integral and an internal component ofmicroprocessor 70 as is commonly found in the class of devices calledmicrocontrollers. ROM 71 and RAM 72 are shown here external to device 70to better explain the way in which the invention is used to advantage.

With renewed reference to FIG. 2 and FIG. 4, an explanation of theoperation of the position-determining circuit as used to advantage inthe present invention will now be made. As was previously explained, theapparatus of the present invention is used to determine a timemeasurement, or the time that the magnet 43 travels in a clockwisedirection among magnet-detecting devices 31, 33, and 32 as the means formeasuring the degree of rotation of 61 and 62.

As rotor disk 40 travels, for purposes of this embodiment in a clockwisedirection, it will encounter magnet-detecting device 31 first. Whenmagnet 43 passes over magnet-detecting device 31, the magnetic fieldproduced by magnet 43 is inductively coupled into magnet-detectingdevice 31, inducing a pulse. Trailing edge detection device 81 detectsthe pulse and sends an output signal to NOR gate 85 and the IRQ input ofmicroprocessor 70. Microprocessor 70 then polls its I/O ports for anyinput signal. Trailing edge detecting device 81 also sends its outputsignal to port PO1 of the microprocessor 70. Microprocessor 70, upondetection of this first detection device signal on PO1, will reset andstart a counting routine. As rotor disk 40 further travels in aclockwise direction, the magnetic field emanating from magnet 43 isuncoupled from magnet-detecting device 31. The counter, however,continues counting.

Magnet 43 next passes over magnet-detecting device 33, where a pulse isinduced in magnet-detecting device 33, causing trailing edge detectiondevice 83 to send its output signal to NOR gate 85 and to port PO2.Microprocessor 70 then polls its I/O ports for any input signal.Microprocessor 70, upon detection of this second detection signal onPO2, will halt the counter routine, read and store the first count inRAM 72, and reset and start the counter routine again. As rotor disk 40further travels in its clockwise direction, the magnetic field emanatingfrom magnet 43 is uncoupled from magnetic detecting device 33. Thecounter, however, continues counting.

When the magnetic field from magnet 43 finally passes overmagnet-detecting device 32, a pulse is induced in magnetic detectingdevice 32, causing trigger device 82 to send its output signal to NORgate 85 and the IRQ input and port PO3 of microprocessor 70. Upondetection of this third detection signal on port PO3, the count will behalted and read and stored in RAM 72. The value stored for the firstcount is used for variable T₁ and the value stored for the second countis used for variable T₂ of the ratiometric algorithm explained earlier.Microprocessor 70 under control of the operating program fetches theratiometric algorithm from ROM 72 and the first and second counts T₁ andT₂ from RAM 72 and calculates "P", or the percentage of shaft travel,and stores this value in RAM 72.

Under control of an operating routine, the apparatus can measure,calculate, and update shaft travel on a periodic basis, replacing theold data with new data. Alternatively, the position of the shaft 11 canbe determined on a demand basis or when requested by a process controlsystem. When the process control system issues a request for the latestvalue of shaft travel, the data is transferred via communication BUS 92to control system BUS 91 and to a central controller of the processcontrol system. The process control system can then translate the valuereceived into an absolute position of the valve shaft position fordisplay to a human operator or used as process variable in a processcontrolling routine. It will be appreciated by those skilled in the artthat the value of shaft travel measured and calculated by the presentinvention can also be output to the controller of a valve shaftactuating device where it is used as a feedback signal indicating thepresent position of the shaft as it is being rotated by the actuator.

The present invention has been described with particular reference tothe preferred embodiments thereof. It will be obvious that variouschanges and modifications may be made therein without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. An apparatus for determining the position of ashaft rotatably displaceable along a longitudinal axis comprising:meansfor producing a magnetic field; first detecting means mounted to saidapparatus in a first position adjacent said rotatable shaft and along arotational center line, said first detecting means arranged to produce afirst detection signal responsive to the detection of said magneticfield; second detecting means mounted to said apparatus in a secondposition adjacent said rotatable shaft and along said rotational centerline, said second detecting means arranged to produce a third detectionsignal responsive to the detection of said magnetic field; thirddetecting means mounted to said rotatable shaft along said rotationalcenter line between said first and said second detecting means, saidthird detecting means arranged to be displaced along said rotationalcenter line in direct relationship to the rotation of said shaft, andsaid third detecting means further arranged to produce a seconddetection signal responsive to the detection of said magnetic field;magnetic field directing means having said means for producing amagnetic field mounted thereon, said magnetic field directing meansarranged to be rotated in a constant velocity along said rotationalcenter line to pass over and couple and subsequently uncouple saidmagnetic field sequentially between said first detecting means, saidthird detecting means, and said second detecting means; counting meansarranged to receive said first, said second, and said third detectionsignals and to produce a first count signal responsive to said first andsubsequently said third detection signals and a second count signalresponsive to said third and subsequently said second detection signal;and position determining means arranged to receive said first and saidsecond count signals, whereby in response to said first and said secondcount signals said position-determining means determines the position ofsaid shaft along said longitudinal axis.
 2. The apparatus of claim 1,wherein said means for producing a magnetic field is a permanent magnet.3. The apparatus of claim 1, wherein said first, said second, and saidthird means for producing a magnetic field is an electromagnet.
 4. Theapparatus of claim 1, wherein said apparatus further includes anelectric motor and said magnetic field directing means comprises:aferromagnetic disk mounted to said electric motor; and said means forproducing a magnetic field is mounted to said disk along said rotationalcenter line, whereby said electric motor rotates said disk in a constantvelocity allowing said magnetic field to travel along said rotationalcenter line coupling and subsequently uncoupling said magnetic fieldsequentially to said first detecting means, said third detecting means,and said second detecting means.
 5. The apparatus of claim 1, whereinsaid apparatus further includes a position-determining circuitcomprising of at least:a microprocessor for receiving said first, saidthird, and said second detection signals; a memory store connected tosaid microprocessor; andsaid counting means is a counting schemecontrolled by said microprocessor and under control of saidmicroprocessor said counting scheme starts a first counting sequencewhen said first detection signal is received, and upon receiving saidsecond detection signal ending said first counting sequence andproducing a first count signal, and subsequently starting a secondcounting sequence and ending said second counting sequence and producinga second count signal when said third detection signal is received,whereby said first and said second count signals are stored in saidmemory store.
 6. The apparatus of claim 5, wherein saidposition-determining means is a position-determining scheme stored insaid memory store, said position-determining scheme including arepresentation of the geometry of said apparatus; andresponsive to saidcounting scheme producing said first and said second count signals, saidmicroprocessor retrieves said first and said second count signals fromsaid memory store and utilizes said first and said second count signalswith said position-determining scheme, whereby said position-determiningscheme calculates the position of said shaft along said longitudinalaxis and stores said position in said memory store.
 7. An apparatus fordetermining the position of a shaft rotatably displaceable about alongitudinal axis comprising:means for producing a magnetic field; firstmeans arranged along a rotational center line, disposed to produce afirst detection signal responsive to the detection of said magneticfield; second means arranged along said rotational center line, disposedto produce a third detection signal responsive to the detection of saidmagnetic field; third means mounted to said rotatable shaft and arrangedalong said rotational center line between said first and said secondmeans, disposed to produce a second detection signal responsive to thedetection of said magnetic field; means for directing said magneticfield, arranged to couple and subsequently uncouple said magnetic fieldsequentially to each of said first means, said third means, and saidsecond means; means for counting arranged to receive said first, saidsecond, and said third detection signals and to produce a first countsignal responsive to said first and subsequently said third detectionsignals and a second count signal responsive to said third andsubsequently said second detection signal; and means for determiningsaid shaft's position arranged to receive said first and said secondcount signals, and determine the position of said shaft along saidlongitudinal axis.
 8. The apparatus of claim 7, wherein said means forproducing said magnetic field is a magnet and said first means iscomprised of a first magnet-detecting device, whereby responsive to saidmagnet passing over said first magnet-detecting device said magnetinduces a pulse in said first magnet-detecting device producing saidfirst detection signal.
 9. The apparatus of claim 7, wherein said meansfor producing said magnetic field is a magnet and said second means iscomprised of a second magnet-detecting device, whereby responsive tosaid magnet passing over said second magnet-detecting device said magnetinduces a pulse in said second magnet-detecting device producing saidthird detection signal.
 10. The apparatus of claim 7, wherein said meansfor producing said magnetic field is a magnet and said third means iscomprised of a third magnet-detecting device, whereby responsive to saidmagnet passing over said third magnet-detecting device said magnetinduces a pulse in said third magnet-detecting device producing saidsecond detection signal.
 11. The apparatus of claim 7, wherein saidmeans for directing said magnetic field comprises:an ferromagnetic diskarranged in proximity to said first, said second, and said third means;and said means for producing a magnetic field is mounted to said diskalong said rotational center line, whereby said disk is arranged to berotated in a constant velocity allowing said magnetic field to travelalong said rotational center line coupling and subsequently uncouplingsaid magnetic field sequentially to said first means, said third means,and said second means.
 12. The apparatus of claim 11, wherein saidapparatus further includes an electric motor and said ferromagnetic diskis mounted to said electric motor and said electric motor rotates saiddisk in a constant velocity.
 13. The apparatus of claim 7, wherein saidapparatus further includes a position-determining arrangement comprisingof at least:a controller for receiving said first, said third, and saidsecond detection signals; a memory store connected to said controller;andsaid means for counting is a counting scheme under control of saidcontroller and said counting scheme starts a first counting sequencewhen said first detection signal is received, and upon receiving saidsecond detection signal ending said first counting sequence andproducing a first count signal, and subsequently starting a secondcounting sequence and ending said second counting sequence and producinga second count signal when said third detection signal is received,whereby said first and said second count signals are stored in saidmemory store.
 14. The apparatus of claim 13, wherein said means fordetermining said shaft's position is a position-determining schemestored in said memory store, said position-determining scheme includinga representation of the geometry of said apparatus; andresponsive tosaid counting scheme producing said first and said second count signals,said controller retrieves said first and said second count signals fromsaid memory store and utilizes said first and said second count signalswith said position-determining scheme, whereby said position-determiningscheme calculates the position of said shaft along said longitudinalaxis and stores said position in said memory store.